The present invention relates to a method for quality control of spot welding in welding machines, particularly used for welding bodies of cans obtained from rectangular sheets of white latten meant to assume a cylindrical tubular shape.
The edges are welded electrically by passing under two rollers which constitute the welding electrodes. A flattened copper wire is interposed between each one of the rollers and the white latten in order to avoid direct contact of the roller with the white latten, with a consequent unacceptable accumulation of tin. Such wire, after being used on both sides, is discarded.
The effect of the heating produced by the welding current and of the compression produced by the rollers converts the edges into a welding bead.
A problem that is strongly felt in the above-cited welding technology is weld quality control. A poorly made weld can in fact cause considerable problems when the completed can is filled.
Various solutions have been proposed in order to detect welding defects on the production line; they are based on measuring the surface temperature, on measuring the thickness of the bead and especially on the measurement of electrical values, such as the welding current and the voltage drop, hereinafter termed welding voltage, that occurs across the weld due to the flow of the current. In this regard, solutions have been proposed which consist in detecting and processing the welding voltage.
The proposed embodiments are currently unreliable for assuredly identifying all the defective welding spots.
The aim of the present invention is to obviate the above-described drawbacks of conventional devices by providing a method, applicable to an apparatus, for quality control of spot welding.
Within the scope of this aim, an object of the present invention is to identify the existence of one or more defective welding spots in the body of the can, and to drive a suitable expulsion device in order to reject it.
A further object of the present invention is to provide the operator with suitable indications which allow him to monitor the operating conditions of the welding machine and optionally return them to optimum conditions by means of appropriate adjustments before said machine begins to produce rejects systematically.
A further object of the present invention is to stop the machine if the reject rate exceeds a certain limit.
Still a further object of the present invention is to achieve the above-cited aim in a simple way which is relatively easy to provide in practice, safe in use, effective in operation and relatively low in cost.
These and other objects are achieved by the present method for quality control of spot welding in a welding machine which comprises a transformer with primary and secondary windings, in which the output of a secondary circuit is connected to arms which support roller-type electrodes for said spot welding, a welding current and an impressed voltage being available on said secondary circuit, and a welding voltage being present between said roller-type electrodes, the method comprising the steps of:
sending to an adder a first voltage signal which corresponds to said welding voltage, acquired by means of a first differential amplifier;
sending to said adder a second voltage signal which corresponds to a voltage acquired by means of a second differential amplifier on a portion of said secondary circuit or on the primary winding of said transformer;
sending to said adder a third voltage signal which is correlated to a voltage signal which corresponds to said welding current, acquired by means of a transducer on said secondary winding or primary winding;
sending to a multiplier a sum signal, processed by said adder, and said voltage signal, said multiplier being adapted to calculate a power signal which corresponds to a welding power;
sending said power signal to an integrator which is adapted to provide an energy signal of said welding spot in integration over a time interval between two successive drops to zero of the welding current and such as to cancel out the reactive energy averaged over said interval; and
sending said energy signal to an evaluation circuit for welding quality control.